Ion mobility spectrometry is a technique that separates and detects electrically charged particles (e.g., ions) that have been sorted according to how fast they travel through an electrical field in a chamber containing a gas, typically at atmospheric pressure. Small ions travel through the gas faster than do large ions and reach the end of the chamber first, with successively larger ions arriving later. Because ion mobility spectrometry only sorts ions by size, and not by their chemical properties or other identifying features, it cannot be used in all cases to make a positive identification of unknown compounds. However, ion mobility spectrometers can be used with certain compounds and can make measurements quite rapidly (e.g., in only a few seconds), therefore making them highly desirable for use in certain applications. For example, ion mobility spectrometers are commonly used to detect explosives, narcotics, and chemical warfare (e.g., nerve and blister) agents.
A typical ion mobility spectrometer comprises an ionization region, a drift chamber, and a detector. The ionization region is located at one end of the drift chamber, while the detector is located at the other end of the drift chamber. The ionization region is typically provided with a radioactive source, such as 63Ni, suitable for ionizing the sample material, although other ionizing techniques may be used. Ions of the sample material from the ionization region are introduced into the drift chamber, whereupon they ultimately reach the detector at the far end. The arriving ions cause the detector to generate electrical pulses which may thereafter be interpreted to form a conclusion about the nature of the sample material.
While ion mobility spectrometers of the type just described work well and are being used, they are not without their disadvantages. For example, ion mobility spectrometers having a single drift chamber cannot readily be used with both positive and negative ions, which limits the utility of such spectrometers. For example, while spectrometers that work with positive ions are useful for detecting drugs, nerve agents, and some types of explosives, they are not effective in detecting most types of explosives and blister agents, because such materials are better characterized by detecting the negatively charged ions they produce. Consequently, two separate spectrometers (e.g., both a positive ion type and a negative ion type) must be used in order to detect both classes of materials.
Partly in an effort to address this limitation, so-called “dual mode” ion mobility spectrometers have been developed. Dual mode ion mobility spectrometers utilize two drift chambers. Thus, a single spectrometer can be used to detect both positive and negative ions. Unfortunately, however, most dual mode ion mobility spectrometers cannot provide for the simultaneous detection of both positive and negative ions. Instead, such designs operate in a pulsed mode, wherein the spectrometer alternately detects positive ions, then negative ions. While dual mode ion mobility spectrometers have been developed that can simultaneously detect both positive and negative ions, they typically require complex apparatus for separating the positive and negative ions before they enter the drift chambers. In addition, such separation apparatus also tends to decrease the sensitivities of the spectrometers that utilize them.
Consequently, a need remains for an ion mobility spectrometer and ion mobility spectrometry method that can simultaneously detect both positive and negative ions, but without the disadvantages associated with prior art systems. Additional advantages could be realized if such an improved ion mobility spectrometer and method provided for increased detection limits and resolution.